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Trisomy 21

Genetics and incidence

Trisomy 21, also known as Down syndrome, is caused by the presence of a 3rd chromosome 21, which usually occurs as a pair. Individuals with trisomy 21 thus have 47 chromosomes instead of the expected 46. Trisomy 21 is generally not hereditary, that is, it is not transmitted within families.

Trisomy 21 affects roughly 1 in 770 babies at birth. Any woman can have a baby with trisomy 21. As shown in the following graph, the risk of having a baby with trisomy 21 increases with the age of the mother. The younger the woman, the lower is the risk.

Probability of giving birth to a baby with trisomy 21 by women's age

Physical features

Individuals with trisomy 21 share certain physical features such as upward slanting eyes, a thickening of the skin at the nape of the neck, a round face, lower muscle tone, and flexible joints. However, like all children, children with trisomy 21 resemble their parents. (e.g., eye and hair colour, etc.).

Health problems

Individuals with trisomy 21 run a greater risk of having health problems such as hearing and eye disorders as well as heart and gastrointestinal malformations that may require surgery, etc. Certain health problems can be mitigated by permanent medical follow-up and early treatment.


Most children with trisomy 21 will be able to speak and walk, but take longer to develop their language, walking, and motor skills. Individuals with trisomy 21 have intellectual disabilities that generally range from mild to moderate, but that can occasionally be severe. The development of children with trisomy 21 is influenced by the help and support they receive from an early age. There are developmental differences from person to person and one environment to another. Prenatal tests cannot predict the intellectual level or the appearance of health problems in children with trisomy 21.

Most individuals with trisomy 21 will require varying degrees of support throughout their lives. Nevertheless, certain adults can hold a job and aspire to a life that is largely independent. When given a fair opportunity, individuals with trisomy 21 possess the resources and potential to develop deep emotional relationships and lead lives that are profoundly satisfying for themselves and their entourage.